You can spend as much as you want on a gun, and quite a bit on chokes and shells.

I've been killing them just fine out to 35 yds with a Mossberg 500, standard full accuchoke and Winchester Super X 3" #5 shells. I generally set up where I can't see much more than 40 yds, so when I see the bird, he should be in range.

Any of the "turkey guns" out there will work. To get the most range, you will need to try some chokes and loads. There are basic guidelines for which kind of shot (lead vs. other stuff like hevi-shot) to use with certain choke constrictions. I'm no expert there though.